The general objective of the proposed research is to delineate the developmental relations between language and cognition. The research will focus on early linguistic and cognitive development by three groups of children: children with William syndrome, children with Down syndrome, and normally developing children. Previous researchers have argued that children with Williams syndrome have language skills that exceed their cognitive skills, whereas children with Down syndrome have cognitive skills that are more advanced than their language skills; in general, normally developing children have equivalent levels of linguistic and cognitive skills. Because of the differences in the general nature of the relations between language and cognition for the three populations, inclusion of all three in a single study provides a unique opportunity to investigate the universality or non-universality of specific relations among language and cognition. The proposed research consists of a five year longitudinal study with supplemental studies conducted at specific points in development. Both observational and experimental methodologies will be used. There are four specific objectives. First, a series of general and specific relations between language and cognition will be examined. Second, the reference of children's earliest words will be explored, using observational, quasi- experimental, and experimental procedures. Third, the development and use of lexical operating principles by children with mental retardation will be considered. Finally, general issues of development by children with Williams syndrome and Down syndrome will be addressed. The research will have implications both for theoretical models of the relation between language and cognition and for the design of early cognitive and language intervention for children with developmental disabilities.